1) Field of the Invention
This invention relates to a tube sheet assembly. In particular the invention is concerned with a tube sheet assembly for a waste heat boiler employed in a chemical process plant and which makes use of metal ferrules for protecting the inlets of the exchange tubes of the tube sheet.
2) Description of the Prior Art
Various types of chemical process plants employ heat exchangers or waste heat boilers for heat recovery and cooling. One example of a chemical process employing a waste heat boiler is a reforming process in which light hydrocarbons are converted into a gas mixture comprising carbon monoxide and hydrogen. In the reforming process the mixture of carbon monoxide and hydrogen is called synthesis gas or syngas. As the syngas is formed at high process temperatures it is necessary to dissipate large amounts of heat. This is often achieved with the use of waste heat boilers.
A waste heat boiler typically includes an inlet chamber into which a hot syngas stream can be fed from a transfer line. From the inlet chamber the syngas passes through exchange tubes extending between an inlet tube sheet and an outlet tube sheet. The exchange tubes are surrounded by circulating water such that the syngas is cooled as it passes along the exchanges tubes. The cooled syngas feeds into an outlet chamber from where it may be fed for further processing or can be subjected to another cooling cycle in a secondary heat recovery system operated in series with the waste heat boiler.
Due to the fact that the syngas entering the inlet chamber of the waste heat boiler will have a very high temperature, all components in contact with the syngas must be protected with thermal insulation. Accordingly the transfer line, inlet chamber and inlet tube sheet are provided with insulating lining, typically in the form of refractory lining.
All parts adjacent the joints between the inlet tube sheet and the exchange tubes are subjected to very severe conditions due to the fact that at these positions the syngas stream at its maximum temperature will be in contact with the inner surface of the exchange tubes. For this reason it is conventional practice to protect these parts with tube inserts, also known as ferrules. Generally the ferrules will be inserted into a tube sheet whereafter an insulation layer will be installed around the ferrules as well as on front of the tubesheet in order to provide insulation.
A problem often encountered with tube sheets is so-called metal dusting which refers to the catastrophic degradation of metals in carbonaceous gases, usually in operating temperatures of between 450° C. and 750° C. These high temperatures of tube sheets are of course a result of the high temperatures of the syngas passing therethrough en route to the exchange tubes.
Various solutions have in the past been proposed for addressing the problems associated with metal dusting of tube sheets. These solutions include providing dual layers of refractory castables having different thermal coefficients, installing felt washers between the refractory castable and the tube sheets, providing all ceramic ferrules as well as to provide ceramic ferrules with inner ceramic sleeves lined with fibre.
It is an object of the invention to provide an alternative tube sheet assembly for addressing the problem of metal dusting encountered with conventional tube sheet assemblies.